1. Technical Field
The present invention relates to a wireless multiple access local area spread spectrum communications network, and, more particularly, to a wireless spread spectrum arrangement supporting a plurality of simultaneous private communications through a central private branch exchange (PBX) that serves to connect a plurality of local users to each other as well as to an external communications network.
2. Description of the Prior Art
A PBX network reduces the number of lines required between a group of local users and a remote communications exchange while supporting a certain number of simultaneous network accesses. Each PBX network makes use of an associated private branch exchange (PBX) functioning as an intermediary routing or switching device, which connects the local users with each other as well as with an external network. Calls made to or from the local network must first access the PBX, which then routes the call to its destination.
Wireless multiple access communications networks have been successfully implemented in certain applications. For example, a mobile radio system simultaneously routes a plurality of private communications through a central base station which performs the switching necessary to interconnect the appropriate users, utilizing, for example, frequency shift keying (FSK) modulation and frequency division multiple access (FDMA). In this regard see, for example, U.S. Pat. No. 4,112,257 issued to E. G. Frost on Sept. 5, 1978. Another radio system is the satellite-based communications system described in U.S. Pat. No. 4,291,409 issued to A. Weinberg et al. on Sept. 22, 1981, wherein a central switching arrangement, phase shift keying (PSK) modulation and spread spectrum multiple access (SSMA) are used.
In a paper by P. Freret et al. entitled "Applications of Spread-Spectrum Radio to Wireless Terminal Communications" in Proceedings of NTC, Nov. 30-Dec. 4, 1980, Houston, Texas, Vol. 4, pp. 69.7.1-69.7.4, a multi-user wireless indoor communications system is disclosed, which features PSKSSMA and a central, but nonswitching arrangement, supporting bidirectional radio transmissions between a local computer terminal user and a central computer. The use of diversity in such system to combat multipath interference is also suggested.
A demand presently exists for the flexibility and convenience of a wireless PBX network; unfortunately, all current PBX networks require physical interconnection. Several previously unsolved problems have precluded the evolution of wireless PBX networks. To begin with, the indoor office environment is particularly hostile to wireless multiple access communication, presenting especially aggravated multipath fading and multi-user interference problems. Moreover, many multiple access techniques that have been used for other wireless applications, such as Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) are difficult to implement and call for additional complex circuitry. The Spread Spectrum Multiple Access technique, whose random multiple access capability permits simple implementation, has been found to tolerate only a low number of simultaneous users in a distributed controlled network, heretofore rendering it impractical for local area network applications. In this regard see, for example, G. L. Turin's article "The Effects of Multipath and Fading on the Performance of direct-Sequence CDMA Systems," in IEEE Journal on Selected Topics in Communications, Aug. 1984, Vol. SAC-2, pp. 597-603.
The problem remaining in the prior art is to provide a wireless PBX network, wherein the problems of multipath fading and multiuser interference are substantially alleviated.